In recent days, in the field of digital magnetic recording and reproducing apparatuses (for example, digital video cassette recorders (hereinafter referred to as VCRs)) which adopt a magnetic tape medium, high density recording and a quick search function have been demanded to attain long time recording and quick access respectively. When carrying out quick access, it is the aim to increase the tape speed and at the same time ensure constant tension control.
In general, for the brushless motor driving device for use in driving both take-up and supply reels, a linear driving based on a sine wave which generates little torque ripple is suited in consideration of variations in tension.
The sine wave driving may be performed by a circuit of a simple structure, for example, by a method of utilizing sine wave outputs obtained from a plurality of Hall elements as coil terminal voltages to be applied to respective phases in the motor. In this method, it is required that an N electrode and an S electrode are alternately provided on the rotor side, to generate a magnetic field in such a manner that a change in magnetic force in response to the rotations of the motor forms a sine wave. The respective outputs of a plurality of Hall elements are amplified by the corresponding Hall amplifiers, and the resulting amplified outputs are applied to coil terminals of the respective phases in the motor.
An example structure of the three-phase brushless motor driving device will be explained below.
FIG. 4 shows the structure of the three-phase brushless motor driving device. In FIG. 4, U, V and W respectively indicate balanced 3-phase windings (driving coil) which are Y-connected on the motor stator side, and Hall elements H.sub.u, H.sub.v, and H.sub.w are provided on the stator side around the circumference of the permanent magnet mounted to the rotor, in such a manner that respective adjoining Hall elements have a phase difference of 120.degree..
The amplitude of a Hall signal is too small to apply a sufficient driving current to the driving coils U, V, and W. Therefore, it is required to amplify the Hall signal by a differential amplifier. In the figure, a driving current supply circuit 411 is composed of three differential amplifiers 418, 419 and 420. The drive current supply circuit 411 supplies a driving current to the driving coils U, V and W by differential-amplifying two output signals of the Hall element, which have opposite polarities.
For example, a voltage U.sub.out to be applied to a U-phase winding in the motor may be obtained by differentiating sine wave signals to be outputted from the Hall element H.sub.u. Similarly, the voltage V.sub.out to be applied to a V-phase winding is also obtained by differential-amplifying a sine wave signal of the Hall element H.sub.v, and the voltage W.sub.out to be applied to a W-phase winding is obtained by differential-amplifying a sine wave signal of the Hall element H.sub.w.
Other than the above method, a method of generating driving voltages U.sub.out, V.sub.out and W.sub.out from signals of the Hall elements H.sub.u, H.sub.v and H.sub.w may be adopted. To be specific, as is clear from the equations U.sub.out =H.sub.u -H.sub.v, V.sub.out =H.sub.v -H.sub.w, and W.sub.out =H.sub.w -H.sub.u, a method of utilizing a difference in output signals of the adjoining two Hall elements as a driving voltage to be applied to each phase in the motor has been proposed (Japanese Unexamined Patent Publication No. 38189/1992 (Tokukaihei 4-38189)).
The respective terminal voltages U.sub.out, V.sub.out, and W.sub.out to be outputted from the driving current supply circuit 411 are sent to a full-wave rectifying circuit 415, and are rectified into one direction. Furthermore, an output from the full-wave rectifying circuit 415 is supplied to a smoothing circuit 416, and is converted into a DC voltage V.sub.mt in which three-phase voltages of the motor are composed.
Then, a control voltage V.sub.ctr from an input terminal 401 of the driving device and an output voltage V.sub.mt from the smoothing circuit 416 are sent to a comparator 402, and an error signal between the control voltage V.sub.ctr and the output voltage V.sub.mt is outputted. The resulting error signal is sent to one of input terminals 404a of a switch 404 and to an inverting amplifier 403. To the other input terminal 404b of the switch 404, an output of the inverting amplifier 403 is sent. The switch 404 is switched between a non-inverting signal of an error signal and an inverting signal of the error signal from the comparator 402, and the switched signal is sent to the Hall element driving amplifier 405.
In the Hall element driving amplifier 405, a non-inverting output terminal 405a is connected to one of the control current terminals of the Hall element via a resistor 406, and an inverting output terminal 405b is connected to the other control current terminal of the Hall element via a resistor 407.
As described, according to the driving device, the output voltage of the Hall element is controlled by the feedback control arrangement for controlling the control voltage to be applied to the control current terminal of the Hall element so that the control voltage V.sub.ctr and the output voltage V.sub.mt of the smoothing circuit 416 are equivalent.
Additionally, the voltage to be applied to the winding of the brushless motor is determined by an output voltage of the Hall element, and the rotating direction of the motor is determined by altering the direction of the control current flowing in the Hall element by inverting the polarity of the error signal to be inputted to the Hall element driving amplifier 405.
However, the conventional brushless motor driving device requires a high closed-loop gain to achieve an improved function of the feedback control. As a result, a response sensitivity of an output signal to the input signal of the driving device increases, and a wide dynamic range of the input signal cannot be ensured.
The conventional brushless motor driving device also has the following drawbacks. As a time constant of the smoothing circuit is fixed, a proportional amount of feedback of the motor terminal voltage cannot be obtained over a range of rotations from low-speed rotations to high-speed rotations, and a non-linear element becomes large.
Furthermore, when driving the driving device at low motor terminal voltage (in a vicinity of 0V!), due to an absence of an error signal, the driving voltage may not be controlled by the control voltage.